The purpose of the study was to evaluate the effect of intravenous mannitol 20% on intraocular pressure in vitrectomised silicone-oil filled eyes. A prospective analysis of consecutive patients was undertaken with their fellow eyes taken as controls. Thirty-one patients were included with mean age of 40.16 years. Baseline parameters were measured which included applanation tonometry, lens status, gonioscopy, optic nerve head status and pachymetry. The intraocular pressure (IOP) was measured at 15 minutes prior and then four more readings were taken at 15 minutes, 45 minutes, two hours and four hours post intravenous mannitol infusion. Statistically significant difference was found with IOP reduction of more than 6mmHg in 30% of cases at all time intervals post infusion. No other correlation was found among other baseline parameters that were measured. Neovascular glaucoma was the only condition that had poor response after mannitol infusion. A conclusion was inferred that short-term IOP reduction could be achieved with intravenous mannitol infusion, although the clarity in the pathophysiology of the effect was not ascertained and various possible means were thought of.